Horizontal axis wind turbines comprise a rotor provided with a number of wind turbine blades—often two or three—which extend radially from a hub. The wind turbine blades have a profile transversely to the longitudinal or radial direction of the wind turbine blades. The wind turbine blades comprise a root region with a substantially circular profile closest to the hub, a blade region, also named airfoil region, with a lift-generating profile furthest away from the hub, and optionally a transition region between the root region and the blade region, the profile of the transition region changing gradually in the radial direction from the circular profile of the root region to the lift-generating profile of the blade region. The lift-generating profile is provided with a suction side and a pressure side as well as a leading edge and a trailing edge. The root region has a root end surface comprising a number of bores arranged in the proximity of the circumference of the substantially circular profile and extending from the root end surface into the root region. The bores each has an inner thread for receiving a fastening means, such as a bolt, from the hub when the wind turbine blade is mounted to the hub. As the demand for more powerful wind turbines, e.g. longer wind turbine blades, is increasing and the demand for more cost-effective wind turbines is increasing at the same time, it has proved difficult to provide wind turbine blades having sufficient structural strength and utilising the material optimally. This applies especially to the root region, which is critical for a secure fastening of the wind turbine blade to the hub during use of the wind turbine.
WO 2010/018225 provides a method of manufacturing a wind turbine blade comprising a steel wire or steel fibre-reinforced polymer matrix. However, the document does not address the problem of how the root region is to be designed to withstand extreme loads in the connection between the blade root and the hub.
WO 03/078832 discloses a wind turbine blade comprising a longitudinally extending transition region, in which the ratio between two types of fibres, e.g. glass and carbon fibres, gradually changes in order to obtain a smooth transition in stiffness in the longitudinal direction of the blade.
GB 2 451 192 describes a wind turbine blade comprising a spar box and a blade shell. The spar box comprises a reinforcement material inlcuding two or more different types of carbon fibres. The proportion between the at least two types of carbon varies in the longitudinal direction so that the elastic modulus of the fibrous composite material increases towards the tip. Further, the blade shell may comprise other types of fibres extending along the entire longitudinal extent of the blade. Metallic fibres such as steel fibres are mentioned as a possibility.